The invention relates generally to operations aiming at the mobility of a wireless terminal in a data transmission network, where data is transmitted in packets, i.e. cells. The invention particularly relates to a method, the application whereof reduces interference in data transmission in a situation where the wireless terminal of an ATM network performs a handover.
The ATM (Asynchronous Transfer Mode) network is a data transmission system where data is transferred in digital form as 53-byte cells from one terminal to another through switches and rapid transfer connections therebetween. Each cell contains a 48-byte payload and a 5-byte header. In order to save space, the header information of each cell does not include complete address information describing the used data transfer route from the transmitting to the receiving device, but only information of the virtual path and channel where said data transfer connection is being carried. The switches or nodes of the network contain necessary routing information, on the basis whereof said identifiers of the virtual path and channel are interpreted as references to the respective node in succession.
It is to be expected that in the future the ATM network, which has so far been based mainly on cable connections, will also serve wireless terminals that are in contact with the network via radio base stations, i.e. access points. These mobile terminals may move with respect to the base stations and their coverage area, in which case the system must be able to execute a handover when necessary. A specific feature of each ATM connection is the contract between the terminal and the network as to the quality of service (QoS) required by the connection. This contract particularly covers the maximum length of the delays allowed in the connection and the capacity needed by the connection, which capacity is measured in units of transfer rate (for instance cells/s). The agreed quality of service for the connection and its upkeep are important factors when making decisions as for the point of time of the handover and the new base station to be assigned for a given mobile terminal.
FIG. 1 illustrates a simple radio extension of an ATM system comprising a mobile terminal 1, three base stations, henceforth called access points (AP) 2, 3 and 4, as well as a switch 5 transmitting connections between the access points and the rest of the network. At first the connection between the mobile terminal 1 and the ATM network proceeds via the access point 2. During connection, the mobile terminal may also have information of the existence of access points 3 and 4 as recorded in a (given register of alternative access points. When the connection to the access point 2 weakens the mobile terminal 1 hands the connection over to the access point 3 or 4. The mobile terminal 1 may also have several simultaneous ATM connections with cell streams that are independent of each other.
It is typical of the ATM system that cells of a given cell stream must not be lost; neither are the cells allowed to double or to change their relative order at the different stages of the connection, which would cause difficult synchronisation demands in the handover. A loss of cells or a confusion in their relative order generally results in that on some higher protocol layer, there is detected an incorrect check sum or other indicator, in which case a certain multi-cell data structure PDU (Protocol Data Unit) is discarded and selected to be retransmitted. This is uneconomical from the point of view of utilising the network capacity.
The handing over of access points in a wireless ATM network has been discussed in prior art publications, but they have usually not dealt with problems connected to the quality of service nor suggested a method for preventing the loss, doubling or disorganisation of ATM cells. In the patent publication EP 426,269 (British Telecommunications) there is known a method where base stations are grouped into groups of several stations. In order to help and speed up the handover, all cells directed to a mobile terminal located in the coverage area of a given base station are transmitted, by the data transmission network, to all base stations of the same group. Said publication introduces a system where the old base station transmits all cells that it has received, whereafter a new base station starts transmitting cells that are transferred therethrough. The publication also introduces a predictor mechanism to be connected to the base station control unit or to the switch controlling the base stations, the task of said predictor being to observe the movements of the mobile terminal from one coverage area to another and to estimate to which area the mobile terminal will move next. The purpose of this system is to reduce the number of cells that are transmitted in vain to the more distant base stations in the group. This application is not, however, capable of maintaining the order of the cells nor preventing the loss of certain cells, because the old and the new base station cannot know exactly which cells were transmitted and received correctly immediately before and/or after the handover.
From the patent publication EP 366,342 (AT & T) there is known a system where data is transmitted in a cellular radio network as cells, and the header of each cell contains an unchanging part which remains the same irrespective of chances in the routing, plus a changing part, the content whereof changes in connection with the handover or some other change in the routing. The publication suggests that the handover can be made easier by means of said system, but only as regards the definition of the routing. A similar system is described in the EP patent publication 577,959 (Roke Manor Research Ltd.), where the focus is particularly an ATM network. Now the unchanging part of the cell header is a so-called VCI (Virtual Channel Identifier) field, and the changing part is a so-called VPI (Virtual Path Identifier) field. Neither of said publications describes a method that could guarantee the maintenance of the order of the cells and/or prevent their loss during a handover.
From the EP patent publication 577,960 (Roke Manor Research Ltd.) there is known an application where at least one of the base stations of the cellular network is, via the ATM network, connected to at least two switches, which in said application also serve as gateway equipment for the ATM network and the cable-transmitted telephone network. The idea is to arrange the route-defining VPI and VCI codes of the ATM network so that although a given mobile terminal moves over to an area covered by another switch (or another mobile phone exchange), the routing is taken care of by linkage through the original switch. This arrangement has certain advantages in order to reduce connections to be switched in the network, but it does not affect possible loss and/or delay of cells taking place in between access points and the mobile terminal during a handover.
The article "BAHAMA: A Broadband Ad-Hoc Wireless ATM Local-Area Network", Proc. ICC '95, 18-22 Jun. 1995, Seattle, by K. Y. Eng et al, describes an arrangement where the GFC field included in the header part of the ATM cells is used for realising a sequential numbering on a per-cell basis. The purpose is to enhance the synchronisation and combining of cell streams arriving via two parallel routes to a given merging point. A particular aim of the numbering of cells is that they could be unambiguously identified, so that the cells would not be doubled or lost when combining cell streams, and that their order would remain unchanged. A new problem could now be that by means of the maximum four bits of the GFC field, only numbers from 0 to 15 can be represented, in which case the numbering cycle becomes so short that successive cells with similar numbers can be confused.
From the Finnish patent application FI 955,812, "Maintaining the composition of transferred data during handover", applicant Nokia Mobile Phones Oy, which is not yet public when the present application is being filed, there is known a method where ATM cells can be referred to with an accuracy of at least a given amount of cells, in which case the base stations can exchange information as to which cells were transmitted and/or received successfully in connection with the handover. In said method, the old base station sends to the ATM switch and/or to the new base station information as to which cells were successfully transferred therefrom to the new base station, in which case the transferring of cells is continued between the new access point and the mobile terminal starting from the first cell which was not successfully transmitted via the old access point. Moreover, the application introduces an acknowledgement system, whereby a given access point and mobile terminal both control which cells were successfully transferred over the radio connection. This method does not take into account the general control of the handover nor the quality of service agreed for the different connections.